The mining and processing of phosphate ores create two types of wastes for which there has previously been no satisfactory use.
One of these wastes is phosphogypsum, which is a by-product of the phosphoric acid production process. For each ton of P.sub.2 O.sub.5 produced, approximately five tons of phosphogypsum are also produced. To this date, more than 700 million tons have accumulated in Florida on at least 5000 acres, and phosphogypsum continues to accumulate there at an estimated rate of about 30 million tons per year. After phosphate rock has been reacted with sulfuric acid to produce phosphoric acid, resultant phosphogypsum is filtered and pumped as a slurry to ponds where it settles. As the gypsum settles out, a dragline moves a portion and deposits it on a dike, raising the height of the wall. This process is repeated and a "stack" is created. As the stack grows in height (up to 200 feet high) the area of the pond decreases until the available pond capacity becomes too small and the pumping height requires too much energy. At this stage therefore, the phosphogypsum stack reaches the end of its useful life and is ready to be closed.
The disposal, storage or handling of these stacks is of considerable concern, because the chemical processing of phosphate rock leaves and concentrates most of the radium, fluoride and traces of heavy metals present in the original phosphate rock, together with decay products such as radon, polonium and lead, in the phosphogypsum by-product. This logically directs attention to construction techniques which minimize the eventual emission of contaminants into structures on reclaimed lands and into the water. The goals of phosphogypsum stack reclamation are to control or minimize any environmental hazards and to improve the aesthetics.
Another of the wastes are phosphatic clays which are separated from the phosphatic ore (matrix) by washing during the upgrading (beneficiation) step. The diluted clay then flows into diked ponds (or settling areas) where the clay is allowed to settle. A survey by the Florida Department of Natural Resources, Bureau of Mine Reclamation, states that, as of December 1991, there were 102,000 acres of active and inactive clay settling areas in Florida, and that 23,000 additional acres were planned. Because of the physical properties of the clays, reclaimed clay settling areas are not suitable for urban or suburban development, However, the clays are highly fertile and thus clay settling areas do have a potential for agricultural uses. The clay settling areas could also serve as stream headwaters and provide wildlife habitat.
Approximately one ton of waste clay is generated for each ton of finished phosphate rock product isolated from the ore. Historically, the waste clay has been disposed of in above ground diked ponds where the diluted clay slowly settles through the action of gravity. This conventional method of handling the waste clays has several disadvantages. It may take up to 20 years before the clays settle from an initial 3 to 5% solids level to a 20% solids level. Even then the clays occupy twice the volume they originally did in the ground. Almost half of the acreage of a phosphate or processing site has to be devoted to storing the diked clays. When the clay ponds have settled to their maximum extent and crusted over, the resulting land has limited utilities. It is restricted to low income-producing applications such as cattle grazing or wildlife habitat. This adversely impacts the tax base and general economy.
The general public, recalling a few sizable retention pond dam breaks, is generally apprehensive about future dam failures, even though today's dams are carefully engineered and maintained. Also, the clays tie up vast amounts of process water for years. A fraction of this water evaporates and is not recovered. Further, an appreciable amount of phosphorous values is present in the suspended solids, and in fact the phosphorous concentration in the suspended solids may be of the same order as the phosphorous concentration in the suspended solids may be of the same order as the phosphorous concentration in the phosphate ore originally mined. Thus, at the present time, a significant amount of phosphorous values is not recovered.
A process for the treatment of phosphatic clay suspensions has been disclosed and claimed in U.S. Pat. No. 4,279,635 (Krofchak) issued Jul. 21, 1981, the content of which are hereby incorporated herein by reference.
This patent describes a process for treating thickened phosphatic clay suspensions produced in the processing of phosphate ore by water beneficiation, such suspensions having a solids content of from about 5% to about 30%. The process comprises mixing a sufficient amount of deflocculating agent with a suspension to cause deflocculation of the suspension to an extent sufficient to reduce the viscosity of the sludge to a value enabling the deflocculated suspension to be readily pumped from one location to another. The deflocculating agent comprises an alkali metal phosphate. The deflocculated suspension of reduced viscosity is pumped from one location to another, and an additive compound comprising an alkaline earth metal oxide or hydroxide is subsequently mixed with the deflocculated suspension in an amount sufficient to cause the formation of large silicate molecules with resultant gelling and setting of the sludge into an inert solid material.
The patent only addresses liquefying the clay sufficiently to pump it to a fill location, where it is solidified with purchased reagents such as lime and cement. The patent does not in any way teach the full scope of problems such as disposal and/or utilization of the huge quantities of phosphogypsum or the manufacture of chemical reagents such as cement and/or acid to recover phosphorous compounds into a saleable form.
It is therefore an object of the present invention to unify all the waste problems associated with phosphate mining in a manner which achieves a more economical solution which the prior patent referred to above does not teach.